Battery chargers typically view a battery as a two terminal device, a positive terminal and a negative terminal. During charging, typical chargers will monitor the voltage across the two terminals and the current entering the battery and the voltage profiles of the battery during charging. For example, one decision made during charging of a NiCd battery is to look for a slight "dip" in the battery voltage. This indicates a fully charged battery.
Lithium ion batteries present a different problem in that these batteries, similar to NiCd batteries, are disposed in series. However, it is very important to ensure that there is no overcharging or overdischarging of a particular cell. A given cell can be overcharged or undercharged and be destroyed. Destruction of a single cell will render an entire battery pack unusable. Therefore, charging systems must be made based upon a given cell and not on the entire group of cells as a battery pack.